In the prior art, efforts were made to minimize or reduce the rate of heat-transfer effects of the molten metal on the casting belts in continuous casting machines in order to protect these wide, thin, revolving flexible casting belts, especially to minimize their distortion, buckling, wrinkling, rippling, or fluting.
For such purposes, the temperature of the flexible casting belts in twin-belt casting machines was controllably elevated prior to contact with the metal being cast. For providing belt temperature elevation, heaters were directed at close range aganist the front (outer) faces of the casting belts before the belts came into contact with the molten metal. Also, hot fluid, such as steam, was circulated within hollow nip rolls at the entrance of the casting region to elevate the temperature of the casting belts. Further, the high velocity liquid coolant which serves to cool the reverse (inner) faces of the casting belts was directed onto these inner surfaces so that this cooling effect occurred only momentarily before or simultaneously with the contact of molten metal against the belt's front faces, as described and claimed in U.S. Pat. No. 4,082,101. Method and apparatus for belt temperature elevation are described and claimed in U.S. Pat. Nos. 3,937,270 and 4,002,197. In addition, casting belts were preheated by direct application of steam to their reverse surfaces before the endless belts entered the casting zone, thereby reducing the differential temperature of the belt before and after it entered the casting zone to thereby reduce distortion. Method and apparatus for such steam preheating are described and claimed in U.S. Pat. No. 4,537,243.
In order to reduce the rate of heat transfer into the revolving belts and travelling edge dams and to improve their durability to withstand thermal and mechanical stresses and to improve the cast product, casting belts and edge dam blocks were coated on their front surfaces with insulative and protective materials, for example as described and claimed in U.S. Pat. Nos. 3,871,905, 4,588,021, and 4,545,423. Pat. No. 4,588,021 describes a unitary-layer matrix belt coating having controlled porosity characteristics fusion bonded to a belt usually cold-rolled from low carbon steel and usually having a thickness in the range from about 0.035 of an inch up to about 0.065 of an inch. Some belts were also made from a titanium-containing steel, as described in U.S. Pat. No. 4,092,155, which is work hardened by cold rolling. The controlled fluid-accessible-porosity characteristics in the fusion-bonded matrix coating was taught as being desirable and important, to the effect that an appropriate level of such porosity contributes substantially to the insulative value and durability of the matrix coating, while at the same time such fluid-accessible porosity enhances the desired characteristics of relative non-wettability of the belt by molten metal. It was believed that this non-wetting enhancement is due in large part to the air retained in the interstitial pores of the porous coating. When molten metal is introduced adjacent to the coated belt, the air in the pores is heated and expands out of the pores and so supplies a gaseous film between the molten metal and the belt coating, thereby preventing the molten metal from wetting the coated belt during the critical initial time when a skin of solidfied metal is being formed on the product being cast in the continuous casting process.
A machine for producing an insulative and protective coating on a wide thin endless revolving flexible casting belt and a thermal spray gun traversing apparatus and system for laterally tracking a revolving casting belt being thermally spray coated are described and claimed in U.S. Pat. Nos. 4,487,157 and 4,487,790.
In U.S. Pat. Nos. 4,593,742 and 4,648,438 described and claimed a method and apparatus for protecting the molten metal surface within the mold cavity from oxygen and other detrimental atmospheric gases, hydrogen or water vapor, sulphuric gases or carbonic acid gas by injecting into the mold and applying to the moving mold surfaces an inert gas. As suitable shielding gases, being inert and essentially nonreactive in relation to the metal being cast, nitrogen, argon or carbon dioxide are described. In addition, it was suggested to use a lighter-than-air gas below the casting metal in the mold and a heavier-than-air gas above the casting metal. As a lighter-than-air gas nitrogen was mentioned, which is about 3 percent lighter than air. As a heavier-than-air gas argon was mentioned, which is about 35 percent heavier than air.
In twin-belt continuous metal casting machines, particularly for casting copper, the travelling side (edge) dams have been formed by stringing slotted damblocks along the entire length of a flexible metal strap, all blocks being free to slide along the strap. These blocks were cooled by controlled coolant sprays in a chamber, their temperature after cooling was sensed, and then an insulative material was applied to the damblocks before re-entry into the casting zone. The damblocks were preferably made of a bronze alloy which presented a better resistance to heat crackling and a higher heat conductivity than the nickel-chromium steel damblocks previously used for casting copper. This damblock alloy was "Bronze Corson," a trademark of Usines a Cuivre et a Zinc de Liege, and has a composition of 1.5 to 2.5% Nickel, 0.4 to 0.9% Silicon, 0.1 to 0.3% Iron, 0.1 to 0.5% Chromium, balance Copper. These damblocks conducted heat rapidly away from the two side surfaces of the cast copper bar product. This method and apparatus for continuously casting a copper bar product using such "Bronze Corson" alloy damblocks is described and claimed in U.S. Pat. No. 4,155,396.
The problems of belt distortion, buckling, wrinkling, rippling or fluting are more pronounced near the entrance to the mold, usually within about 15 to 20 inches (about 38 to 51 centimeters) of the line of tangency of the belt with the pulley roll at the mold entrance, as is illustrated in FIG. 8 of U.S. Pat. Nos. 3,937,270 and 4,002,197.
In casting Aluminum, the experience over the past several years has shown that relatively pure Aluminum and Aluminum Alloys having narrow ranges of solidification temperatures, i.e., narrow ranges of no more than about 15.degree. C. are continuously castable in twin-belt casting machines to commercially acceptable specifications without undue complications. However, Aluminum Alloys having wider ranges of solidification temperatures above about 40.degree. C. are found to be much more difficult to continuously cast to commercially acceptable specifications.
All of the above referenced patents are assigned to the same assignee as the present application. The disclosures of all of the foregoing patents are incorporated herein by reference.